Реферат: Respiration and Respiratory Systems
Respiration and Respiratory Systems
LUNG CANCER
Up to the time of World War II, cancer of the lung was a relatively rare
condition. The increase in its incidence in Europe after World War II was at
first ascribed to better diagnostic methods, but by 1956 it had become clear
that the rate of increase was too great to be accounted for in this way. At
that time the first epidemiological studies began to indicate that a long
history of cigarette smoking was associated with a great increase in risk of
death from lung cancer. By 1965 cancer of the lung and bronchus accounted for
43 percent of all cancers in the United States in men, an incidence nearly
three times greater than that of the second most common cancer (of the
prostate gland) in men, which accounted for 16.7 percent of cancers. The 1964
Report of the Advisory Committee to the Surgeon General of the Public Health
Service (United States) concluded categorically that cigarette smoking was
causally related to lung cancer in men. Since then, many further studies in
diverse countries have confirmed this conclusion.
The incidence of lung cancer in women began to rise in 1960 and continued
rising through the mid-1980s. This is believed to be explained by the later
development of heavy cigarette smoking in women compared with men, who
greatly increased their cigarette consumption during World War II. By 1988
there was evidence suggesting that the peak incidence of lung cancer due to
cigarette smoking in men may have been passed. The incidence of lung cancer
mortality in women, however, is increasing.
The reason for the carcinogenicity of tobacco smoke is not known. Tobacco
smoke contains many carcinogenic materials, and although it is assumed that
the "tars" in tobacco smoke probably contain a substantial fraction of the
cancer-causing condensate, it is not yet established which of these is
responsible. In addition to its single-agent effects, cigarette smoking
greatly potentiates the cancer-causing proclivity of asbestos fibres,
increases the risk of lung cancer due to inhalation of radon daughters
(products of the radioactive decay of radon gas), and possibly also increases
the risk of lung cancer due to arsenic exposure. Cigarette smoke may be a
promoter rather than an initiator of lung cancer, but this question cannot be
resolved until the process of cancer formation is better understood. Recent
data suggest that those who do not smoke but who live or work with smokers
and who therefore are exposed to environmental tobacco smoke may be at
increased risk for lung cancer, eloquent testimony to the power of cigarettes
to induce or promote the disease.
Because lung cancer is caused by different types of tumour, because it may be
located in different parts of the lung, and because it may spread beyond the
lungs at an early stage, the first symptoms noted by the patient vary from
blood staining of the sputum, to a pneumonia that does not resolve fully with
antibiotics, to shortness of breath due to a pleural effusion; the physician
may discover distant metastases to the skeleton, or in the brain that cause
symptoms unrelated to the lung. Lymph nodes may be involved early, and
enlargement of the lymph nodes in the neck may lead to a chest examination
and the discovery of a tumour. In some cases a small tumour metastasis in the
skin may be the first sign of the disease. Lung cancer may develop in an
individual who already has chronic bronchitis and who therefore has had a
cough for many years. The diagnosis depends on securing tissue for
histological examination, although in some cases this entails removal of the
entire neoplasm before a definitive diagnosis can be made.
Survival from lung cancer has improved very little in the past 40 years.
Early detection with routine chest radiographs has been attempted, and large-
scale trials of routine sputum examination for the detection of malignant
cells have been conducted, but neither screening method appears to have a
major impact on mortality. Therefore, attention has been turned to prevention
by every means possible. Foremost among them are efforts to inform the public
of the risk and to limit the advertising of cigarettes. Steps have been taken
to reduce asbestos exposure, both in the workplace and in public and private
buildings, and to control air pollution. The contribution of air pollution to
the incidence of lung cancer is not known with certainty, though there is
clearly an "urban" factor involved.
Persons exposed to radon daughters are at risk for lung cancer. The hazard
from exposure was formerly thought to be confined to uranium miners, who, by
virtue of their work underground, encounter high levels of these radioactive
materials. However, significant levels of radon daughters have been detected
in houses built over natural sources, and with increasingly efficient
insulation of houses, radon daughters may reach concentrations high enough to
place the occupants at risk for lung cancer. A recent survey of houses in the
United States indicated that about 2 percent of all houses had a level of
radon daughters that posed some risk to the occupants. Major regional
variations in the natural distribution of radon occur, and it is not yet
possible to quantify precisely the actual magnitude of the risk. In some
regions of the world (such as the Salzburg region of Austria) levels are high
enough that radon daughters are believed to account for the majority of cases
of lung cancer in nonsmokers.
Workers exposed to arsenic in metal smelting operations, and the community
around the factories from which arsenic is emitted, have an increased risk
for lung cancer. Arsenic is widely used in the electronics industry in the
manufacture of microchips, and careful surveillance of this industry may be
needed to prevent future disease.
Some types of lung cancer are unrelated to cigarette smoking. Alveolar cell
cancer is a slowly spreading condition that affects men and women in equal
proportion and is not related to cigarette smoking. Pulmonary adenocarcinoma
of the lung also has a more equal sex incidence than other types, and
although its incidence is increased in smokers, it may also be caused by
other factors.
It is common to feel intuitively that one should be able to apportion cases
of lung cancer among discrete causes, on a percentage basis. But in
multifactorial disease, this is not possible. Although the incidence of lung
cancer would probably be far lower without cigarette smoking, the
contribution of neither this factor nor any of the other factors mentioned
can be precisely quantified.
